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Studies On Removal Of Mixed Plume Formed By Benzene, Toluene And Chlorinated Ethylenes In Groundwater

Posted on:2011-10-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y L LiuFull Text:PDF
GTID:1100360302492840Subject:Environmental Engineering
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Multi-components plume in groundwater which blends chlorinated solvents with petroleum hydrocarbons has recently attracted great attention because of its well-known carcinogenicity in humans and pronounced difficulty to be remediated simultaneously by a single technology. Chlorinated solvents such as perchloroethylene (PCE) and trichloroethylene (TCE), can be dechlorinated rapidly by granular iron; petroleum hydrocarbons which mainly refers to BTEX (benzene, toluene, ethylbenzene and xylenes), can be biodegraded or biotransformed by anoxic or anaerobic processes. Thus, it is valuable for setting up sequenced units combined a granular iron permeable reactive barrier (Fe0-PRB) with an anoxic wall to rescue such plumes in aquifers. However, the linkup between the chemical and bio-treated zone that is the most key technology hasn't been well understood; and firstly, two essential issues should be taken into account: (1) how does BTEX influence the long-term performance of an Fe0-PRB? and (2) can the aborigines survive and grow in the severe surroundings downgradient of the Fe0-PRB?This dissertation presents three-group-column experiments which were conducted to evaluate the effects of benzene, toluene on the long-term performance of reductive dechlorination of PCE and/or TCE by granular iron. Moreover, three classes of microbes which fed with benzene and/or toluene and cultured without extra nutrients and electron acceptors in anoxic condition were inoculated from soil chronically contaminated by cruel oils. Laboratory batch studies were performed to assess the potential for biodegradation of benzene and toluene in the presence of TCE or cis-1, 2- DCE at high pH levels.The results showed as follows:1. The kinetics of PCE and TCE (at the initial concentration of 2 mg.L-1 more or less) reduction was accorded with pseudo first-order even in the presence of benzene or toluene (at about 2 mg.L-1, respectively). After benzene or toluene reached adsorption equilibrium in the reaction columns, the presence of benzene increased PCE degradation by 18.0% averagely; otherwise, in the existence of toluene, PCE reduction rates decreased by 12.3%. Nevertheless, the presence of benzene or toluene both inhibited the removal of TCE by 17.5% and 22.9%, respectively. When PCE and TCE were mixed, the results suggested that benzene would be a potential promoter, averagely increased PCE and TCE degradation by 8.9% and 7.2%, respectively; but toluene suppressed the removal of the mixture, averagely declined the removal rates by 20.7% and 13.1%, respectively.2. The investigated changing of groundwater chemistry in the presence of PCE and/or TCE showed noticeable disappearance of dissolved oxygen (DO), calcium(II), bicarbonate and sulfate occurred and pH increased to 9.6. The great calcium carbonate minerals accumulated within all the columns. However, in the presence of benzene or toluene, there were negligibly various among these changes.3. The daughter products of PCE or TCE in the presence/absence of benzene or toluene were identical. The presence of benzene or toluene affected the maximum accumulation of cis-1, 2-DCE within the columns, though; they did not influence the efflux concentrations of cis-1, 2-DCE.4. Mineral precipitations were still the definitive factor that influenced the long-term performance of an Fe0-PRB.5. Benzene and toluene were rapidly degraded in anaerobic conditions with Fe(III) as an electron acceptor by the three enriched cultures, B-bacteria, T-bacteria and M-bacteria, respectively. All the cultures were capable of degrading benzene and toluene completely. The degradation processes fit first-order kinetics equations well and the biodegradation rate constants range from 0.43 to 0.49 d-1.6. The two predominant benzene- and toluene-degrading bacteria were Bacillus sp. and Xanthobacter autotrophicus sp. at alkaline conditions. The alkaliphiles of the three cultures could grow well at the initial pH 10.6.7. The presence of TCE inhibited substrate degradation. At pH 8.7, benzene degradation by B-bacteria was severely inhibited in the presence of TCE (100μg.L-1) and its half life increased more 3-4 times longer than in the absence of TCE. The slightly increasing inhibition was observed in the presence of TCE (500μg.L-1). On the contrary, the presence of TCE (100μg.L-1) had no effect on toluene degradation by T-bacteria nearly; the significant inhibition was observed when increasing TCE concentration to 500μg.L-1 and its half life delayed more 3-4 times longer than in the absence of TCE. As far as M-bacteria degrading benzene and toluene, the inhibition in the presence of TCE was different from neither B-bacteria degrading benzene nor T-bacteria degrading toluene. The presence of TCE (100μg.L-1) delayed half lives of interest by about 0.5 times. Increasing TCE concentrations to 500μg.L-1 led to the gradually increased inhibition. The effect of TCE on the three cultures at pH 10.6 was very similar with that at the pH 8.7.8. The degradation of benzene and/or toluene by the three cultures was not almost inhibited in the presence of cis-1, 2-DCE (80μg.L-1) at the initial pH levels of 8.7 and 10.6, respectively.In conclusion, in design of sequenced anaerobic PRBs for the in situ restoration of TCE-, benzene- and toluene-contaminated aquifers, the results suggested that: (1) it should be not necessary to increase the width of the Fe0-PRB upgradient of the sequenced units to meet the remedial goals or to thoroughly remove all the chlorinated daughter products of TCE; (2) no extra pH buffer zone between an Fe0-PRB and a bio-wall should be essential; and (3) considering the bacterial activities, it let low concentrations of cis-1, 2-DCE and TCE pour into the downgradient bio-barrier over time.The effect of their cometabolism intermediates (such as vinyl chloride) on the growth of bacteria needed further investigation.
Keywords/Search Tags:Benzene, Toluene, Perchloroethylene (PCE), Trichloroethylene (TCE), Cis-1, 2-dichloroethylene (cis-1, 2-DCE), Granular Iron, Permeable Reactive Barrier (PRB), Biodegradation
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